https://www.gamespot.com/articles/last-chance-free-pokemon-legendary-available-for-u/1100-6459355/

We're well into June, and the start of a month means another free Legendary Pokemon is now available for Ultra Sun and Ultra Moon. The Pokemon Company is continuing its Year of Legendary Pokemon giveaway with another limited-time freebie for all seventh-gen Pokemon games, and this time it's another Legendary from the Kalos region: Zygarde. [Update: This promotion doesn't run for the entire month; it expires on June 24, giving you only a little more time to secure a code for your freebie.]

Unlike all of the past distributions this year, which offered a different Pokemon depending on which version you're playing, all players will receive the same Legendary this time around. What makes this Zygarde special, however, is that it's a Shiny, which is very difficult to obtain through normal play.